Spark plug



July 16, 1940. Q BERSTLER 2,208,178

SPARK PLUG Filed April 15, 1939 INVENTOR.

C f/ i BerJf/er BY A TTORNE Y.

Patented July 16, 1940 UNITED STATES PATENT OFFICE SPARK PLUG Reading,Pa.

Application April 15, 1939, Serial No. 267,995

17 Claims.

My invention relates to spark plugs, and particularly to novel means andmethods controlling the temperatures and improving the operatingcharacteristics thereof.

In internal combustion engines, the firing of the compressed gas chargeshas always been a problem, and particularly upon the introduction ofmodern high-compression engines, diihculty has resulted from the heatingof the spark plugs,

more at certain speeds and loads than at others.

With hard driving, the standard plugs soon lose effectiveness to suchextent as to materially reduce the power and pickup of a car that, if itwere not for this feature, would operate, as it does while it is new,for a much longer period of time.

A great many suggestions have been made for overcoming this objection,including the making of plugsof greatly reduced size, producing themfrom expensive materials, providing them with cooling fins, and alteringthem otherwise.

Plugs of standard makes, in regular commercial use, have the commonfeature of an inner end tip, or nose, projecting toward, or through, theinner end of the shell, in close fitting relation to the centerelectrode, and spaced radially inwardly from the shell.

These tips become incandescent in use, resulting in fusion andvolatization of the metal and dielectric parts, causing excessivecombustion inside the plugs, but outside the insulators, and quicklyimpairing the usefulness of the plugs.

Certain of these, and other, former plugs, in particular service, aretoo cool, which causes them to quickly build up carbon deposits, and toreduce the intensity of the sparks, or short circuit the spark gaps.

From these facts, for a spark plug to have eflicient operation and longlife, it should be neither too hot nor too cool, but should effect abalance between these extremes over a wider range of operatingconditions, such as the speed and load, and the construction andoperation of the engine in which it is employed.

It should be hot enough to burn, or prevent, carbon deposits, but notsubstantially hotter than this, and a plug having such operation over awide range of operating conditions, as does the plug of my invention,may be characterized as the most effectively cool plug.

Another feature common to standard plugs is the corrugated, or ridged,shape of the outer ends of the insulators, ostensibly to provide surfacecreepage distance, but apparently having little value for this purpose,and being of easier breakage, as well as unsightly.

The metal shells of commercial spark plugs have assumed efiicientstandard form, in which various modified porcelain shapes have beentried. However, these so-called porcelains must be constructed to avoidthin sections, sudden changes in section, and other features, wherebythey may be assembled under comparatively great pressure, in the metalshells without breakage, or the introduction of minute cracks, fissures,or weakness resulting in early failure of the plugs.

Many other shapes have also been suggested, with the result ofapparently nothing better on the market than the plugs of simplestandard well-known makes having the aforementioned tips and ridges onthe insulators.

An object of this invention is to overcome the above-mentioneddifficulties, and to provide a spark plug that will operate more nearlyat the critical, or correct, temperature than those heretofore employedand suggested.

Another object of the invention is to provide a spark plug that willincrease the horsepower and driving torque, and improve the generaloperating efficiency of an automotive engine.

Another object of the invention is to prevent the local concentration ofexcessive heat in a spark plug, as well as the general undue heating ofplugs as heretofore constructed, and to provide a plug that will operateat more nearly the critical temperature, which is not hot enough tocause preignition, or to impair the operating efficiency of an engine,or which is not cool enough to cause fouling and failure of the plugs.

Another object of the invention is to provide a pocket space or chamberin the inner end of a spark plug insulator, as distinguished from theinner end of the plug generally, of sufiicient depth and capacity, andof other novel character, to provide for the compression or receptiontherein of a substantial volume of gas sufficient to provide for theremoval of dead air from, and the supply of a good gas mixture at, thearcing zone, and to avoid excessive difierences of temperature betweendiiferent regions of the pocket space.

Another object of the invention is tocontrol the depth or capacity ofpocket spaces of the above-indicated character, in different plugs, inaccordance with compression ratio, whereby, as this ratio increases,such capacity is decreased, and, as the ratio decreases the capacity isincreased.

Another object of the invention is to reduce the compression and flashvolume inside a spark plug, and particularly outside the insulator inthe plug.

Another object of the invention is to take advantage, in a spark plug,of the principle that, to have an insulator surface substantiallyconform to the electrostatic field, will enhance the operation andappearance of the plug, and render it unnecessary to providesurface-creepage ridges of difficult manufacture and ready breakage.

Another object of the invention is to provide spark-plug insulators ofthe herein indicated character that are as readily adapted for use instandard shells as the former porcelains, and to provide plugs that areas 'well'adapted to standard engines as the former plugs.

Another object of the invention is to provide a spark-plug insulatorthat is of easy manufacture by existing methods, that is of readyassembly with a shell under great pressure, without undue danger ofinjury, that is free from delicate or readily-breakable parts for damagein manufacture, handling, shipping or operation, and that will performits function for a long -period of time. I

Another object of the invention is to providea ground electrode for 'a-'-spark plug that resists travel of the are from the intendedarcingzone to the shell on which the electrode is mounted.

Another object of the invention is to provide a method of controllingthe temperature and the operating characteris'ticsoi "spark plugs.

A further object of the invention is to :provide a spark plug, of thecharacter herein set "forth, that is-s'imp'l'e and durable inconstruction, economicalto manufa'cture, and efiective in its operation.

With such objects in 'view, as-well;as other ad vantages whichmay beincident to a utilization of the" improvements, the invention comprisesthe elements and combinations thereof herein set forth and claimed, withthe understanding that the several necessary elements, constituting thesame, may be 'varied inproportion, arrangement, texture and otherfeatures, without departing fromthespirit and scope of the inventionzasintended, and as set forth in the "drawingpspecification'and claims.

In order to render theinvention more clearly understood, means are shownin"the:accompanying drawing :for carrying the same. into practicalefiect, i'without limitingthefinnorovements, :in the useful applicationsthereof, to the particular constructions-shown and described, which aregiven merely byway'of example.

In the-drawing:

.Figure .1 is a'viewyon-an enlarged scale, :par- .tiallyiin sideelevation and partially in e'longi- .tudinal 'icentral-iplan'ez-section, 20]? a spark plug embodying iparts of the invention Fig. '2is a view, similar to the lower portion .of ll, of the invention in-.another form, "or stage;

Fig.3 is'a view, similar "to Fig. 2,.ofztheinventionzinanother form;

a4 is a view, similar to :Fig, 3, -'of the "invention in ax-furtherform; I

Fig. 5 is a contour diagram, in theznatureof a longitudinalcentral-planetemplet. or patternot the: lower end "of the plug, :more nearly as shownin Fig.l,*'but. larger; andlhaving imposedthereon certain dotted linesindicating "the above-mentioned tipfeature of former practice; and

Fig. 6 is a similar diagram of portionsof the upper, orouter,'end'length'of the :plug, on'an enlarged" scale, illustrating theI'EIQtiOIl'TOf certain structural v:parts *to the electrostatic field inthis region.

In practicing the invention, the longitudinally inner, or firing, end ofthe plug insulator, instead of projecting toward the combustion chamber,or are zone, as a tip or nose, closely surrounding the center electrodein radially spaced relation to the shell, as in standard practice, isformed as a concavity, like a cup; or pocket, having its "innerlongitudinal sides in free-space relation to the inner electrode.

This inner end pocket provides the free space around the electrode for adistance therealong at least sufficient to provide proper surfacecreepage from the inner electrode to'the shell, or to the groundelectrode. The pocket may cause compression of inert gas, or dead air,around the electrode, or otherwise prevent the imposi-- tion of theintense fire of combustion against the bottom and side portions of thecup, or subject these portions to fire or heat only sufficient tomaintain these parts clean.

Thesdepth and capacity of .this'cup are suflicient to provide for thecompression, .or reception, in the cup .Of .asubstantial volume of gas,and to avoid excessive diiferences of temperature between diiferentregions therein. In other words, in accordance as the compression ratiois large or small, the space depth and volume of the pocket will-besmall or large, respectively,"whereby to remove any dead :air from theregion .of the arc gap, to provide a proper .gas mixture at this point,to provide sufficient heat at vall parts of the pocket to avoid theformation of carbon, and'to prevent .the inner recesses .of the pocketfrom becoming suificientlyacool to allow the collection of :carbondeposits.

Other considerations :in the formation of the cup, or pocket, are thecross section, strength,

texture and manufacture thereof, such that it will not breakreadily fromcontact with other objects, :or from suddenchanges in temperature, asoccurs between sections of vitreous elements, in whichthe change insection between adjoining parts is too-sudden.

' The pocket is additionally shielded, both mechanically and thermally,-.as :by having :theshell extend inwardly of the plug to a position.close to, orlbeyond, itheinnerzend of the cup in prede- 'terminedlateral relation thereto, and by this feature, plus a flange at theinner end of the shell extending radially ,OI laterally inwardly overthe inner periphery'of the cup and, by thelatter feature, plus thefeature of having the ."flange spaced longitudinally-of the cup-or, inany-form, by the {form and;position of the ground-electrode.

Thus they above-mentioned nose-or I tip, ,as provided heretofore, andwhich is a point of high local heat concentration,is:eliminated, and theproper surface-.creepage distance obtained between the electrodes at thearczone, by means which operates .at considerably reduced temperature,

.By the'propercorrelation of the parts, as herein pointed out, thefiring'end of the plug is thus rendered as cool-as-desired, but "stillhotenough to prevent the formation of carbon.

mass of porcelain required, and thermally and electrically to controlthe operating temperature and other characteristics of the insulator.

Referring to- Fig. 1, a plug of the invention comprises, in general, aninsulator A, an inner electrode B having an outer terminal C, and ashell D, on which a ground electrode E is supported.

The insulator A comprises a main body portion 5, an outer-end length 6,and an inner-end length I, which, as shown, are all in one homogeneouslyintegral piece of vitreous dielectric material, such as the usualporcelain, although these parts, or portions thereof may be madeseparately, of the same or different material, and rendered operativelyintegral in other ways, as by glazing them together.

The shell D, as shown, is like, or similar to, a shell of well-knownkind, including a main body portion II), a polygonal, or nut-like,portion I I, an outer-end flange I2, and a tubular inner-end portion I4,of reduced diameter, having exterior screw threads I5, and a radially orlaterally inwardly extending inner-end flange I6, between which, and theportion I4, the ground electrode E is supported, as in a usual manner.

The porcelain body portion 5, which is of larger diameter than thediameters of the lengths 6 and I, is of such diameter as to allow forplus or minus tolerance in manufacture, or baking, and still relativelyclosely fit the shell body I9, with a bottom shoulder I8 over a gasketI9 on a shoulder 20 of the shell D, and a top shoulder 22 under a gasket23, which is held in place by the flange l2.

As shown, the shoulder I8, the gasket I9 and the shoulder 29, are flat,radially of the plug, but may be of other form, and the flange I2, thegasket 23 and the shoulder 22, slope relative to a radial plane of theplug to, in this instance, facilitate and ensure proper assembly, bypressing the flange to the position shown. The gaskets may be made ofcopper or other material.

The arrangement of parts issuch that, with all allowances formanufacturing tolerances, the body portion 5 will be tightly heldbetween the flange I2 and the shoulder 20, and effectively sealed by thegaskets 23 and 20, without danger of crushing the inner-end length Iagainst the flange I6. The latter is therefore in freely-spaced relationto the porcelain, longitudinally of the plug. by a distance F, see Fig.5, which may vary slightly in different plugs.

The inner-end length I which, as shown, has a substantially uniformcylindrical. outer surface laterally spaced from the shell portion M, isprovided with an inner-end portion G of inverted substantially cup orpocket-shape, providing a free space H, radially of the longitudinalaxis of the plug, for a distance J lengthwise of the plug. This distanceis, at least, sufficient to provide proper creepage surface, from apoint 25, or adjacent thereto, on the electrode B, along a transversebottom surface 25 of the cup G, and along an inner longitudinal surface21 of the cup, to the radially and longitudinally innermost edge 28 ofthe cup.

Another consideration, determining the length of the surface 21, or thevolume of the cup space is, as stated above, the compression ratio ofthe engine in which the plug is to be employed. If the cup is deeperthan this, it will be too cool near its closed end, with the resultingcarbon deposits, or fouling, and, if the cup is too shallow, it will notallow the dead air to be displaced far enough from the arc gap toprovide for effective firing.

Where it may be desirable to have a conducting part of, or elementaround, the electrode near the point 25, or at the surface 26, such as awasher-like seal closing the slight space between the electrode B andthe insulator A, which is normally filled with cement, the distance Jwill be sufficient to provide the proper surface creepage distance alongthe surface 21 or other- Wise. The edge 28 may be rounded, as shown, toincrease the creepage path length, and to reduce heating, or thelikelihood of chipping.

The distance J, as shown, is also such that the transverse or closed endof the cup, at the surface 26, is disposed a substantial distance K(Fig. 1) from the insulator body portion 5, and also a substantialdistance from the shell body portion I 0. and from the mouth end of thecup, thereby avoiding thin section of porcelain over a great length, andfacilitating manufacture.

To further assist the latter feature, the surface 21' is also sloping,as shown, or otherwise arranged to increase the thickness of thelongitudinal side wall of the cup G from this thickness L, at the mouthend, to the thickness M, at the inner end of the inner side surface ofthe cup. The distance H, along a radius of the cup, as shown, is greaterthan the radius of any other inner dimension of the insulator, andgreatest near the inner periphery of the inner open end of the cup.

In Figs. 2, 3 and 4, parts corresponding to parts above set forth aredesignated by corresponding reference characters having the suffixes(a), (b) and respectively.

In Fig. 2, the structure is similar to the structure of the lowerportion of Fig.1, except that the pocket Ga is deeper than the pocket G,and

.smaller in diameter at its closed end, and the flange IE0. is thinner,and of larger internal diameter, thus modifying the mass, contour andlocation of the flange relative to the flange IS. The inner-endinsulator length Ia, the tubular shell portion Ida, the threads Ia, andthe ground electrode Ea, may be the same as above set forth. In Fig. 3,the cup Gb of the length 1b has an inner surface of different shape,such as curved or parabolical, and the shell portion Mb, having threadsI51), is provided without a flange corresponding to the flange IE orH511, and has its inner edge closer to the mouth of the cup.

Also, as'shown in this figure, the ground electrode Eb is made thinner,as by flattening, in its longitudinal length 29, and this lengthmaintained further from the electrode Bb, or equivalently arranged, toreduce the conducting surface adjacent to the mouth end of the cup Gb toaccentuate the arcing stress, or localize it, at the intended arcingzone adjacent to the active ends of the electrodes, to reduce theformation of carbon on the insulator or other parts adjacent to thepoint of juncture of the electrode Eb with the shell, and to resisttravel of the are from the arcing zone to the shell.

In Fig. 4, the cup Go of the length 'Ic has an inner surface of furtherdifferent shape, including a ridge 32 adding further creepage distance,and the shell portion Mc, having threads I 50, is also provided withouta flange corresponding to the flange I6 or Ilia, and has its inner edgebetween the ends of the insulator.

The electrode Ec has a portion 290 joined to the arcing portion properthereof through a bend or curve 42, which tends to maintain the are atthc'arcing zone, and to resist its travel from this zone through theincreased distance on the -electrodeto the shell.

Any of the ;features.shownrin:any of'the figures maybe empioyed-in theplace 01" the correspond ing feature in the device of any other figure.

t .e method and structure, illustrated collec tively by Figs. .1, .2, 3,and;e,-provi-de for control ling the operating temperatures andcharacteristics :of spark plugs, by adjusting the -masses,. contours andlocations of the inner-end lengths of the insulators around the innerelectrodes to maintain tree spaces longitudinally'oi the plugs iorpredetermined. distances, corresponding to the distance ll of Fig. 5,from the inner ends of such lengths, by'the distance H, from saidlengths to the innc electrodes, to provide for the com- .pression ofsubstantial volumeof gas in such spaces,,and to avoid excessivedifferences of temperature between different regions thereof, andadjussing the masses and contours of the metal of, or on, the shellsfrom positions between "the of the inner-end insulator lengths, and ofthe metal-of, or associated with, the shell is made at -.a locationwithin'the range indicated by Figs. 1, 2, 3 and 4, and as illustrated bya-line N in v5 having its lower inner end opposite'a point correspondingto the inneredge of the flange it offig. l, and its upper outer endcorresponding to the bottom of the shell portion Me in Fig. 4; .thecongeneric parts of Figs. 2 and 3 being between these limits.

Dotted lines 33, in Fig. -5, indicate the locus of :the outer surface ofa solid'of revolution about the longitudinal axis of the electrode B, ofthe above-mentioned tip or nose, common to standard prior plugs, and"causing. the heating above mentioned. These tips project inwardly, ordownwardly, as indicated, various distances and have'shapes other thanthose shown.

:By a comparison of the lines 33 with the lines :2fi1and 21, thedifference is more readily seen between a protrusion or convexity of theporcelain at this point, as in the prior plugs, and a concavity orpocket, as in the invention hereof. This difference, irrespective of thesmallness of the parts as actually constructed, removes a substantialproportionor mass of porcelain from a criti cal :zone of heating, andsubstitutes 'free space in this region. I I'he protrusion or tip,represented by the lines by its tapered or pointed shape, its positionon the electrode, and its spacing from the shell,

causes'compression and combustion outside the insulator, whereas, in theplug'of the present invention, this effect is removed from the outside,

andp'laced inside, the insulator cup G.

' Further, in addition to the removal of the above mentioned mass, thesubstitute mass, represented by the sidesurface of the cup, is placed ina substantially cooler zone protected from heat by the side walls of thebottom inner-end surfaceof the length 'fi substantially conforms to pthe electrostatic field.

Dyna-mometer and service tests havedemonstrated that the plugs of myinvention produce increased driving torque and horsepower of an engineover a wide range 01 speeds, give an 'automobile smooth pick up in highgear at low speeds, maintain the insulators free from carbon, and the:elctrodesfrom damage, longer than other plugs, and that theyareotherwise a :distinctradvance in theirfield.

The expression oneepiece insulator, as-employed in thespecificatio'nland claims, is to be 1 understood as definingahomogeneously integral piece, as set forth, or :an equivalent unit madeup'cf a plurality-of identifiable, or originally separate elements,- asdistinguished :from ;a plurality of separate insulators which,irrespective of -co- 3 operation thereof with each other, do not make upsuch unit.

I claim as my invention:

i. In a spark plug, an inner electrode, arr-outer en lengthsprotrudingfrom the shella-nd an inner end cup enclosingua free space around-andextendingradially to, said electrode, the inner :end of the shellenclosing a .ireespace around said cup and havinglits smallest insidediameter no less than the-largest inside. diametei of said cup.-

' metal shell, and -a one-piece insulator surrounding said-electrode in.the shell having an outer end length protruding from theouter endof theshell and an inner end cup protruding-from the inner end of the shellenclosing a free space around, and extending'radially to, saidelectrode,

the inner endof the shell enclosing a freespace around said cup.

4. In a spark plug,.a shell, a one-piece insulator in the shell, and aninner electrode extending through the insulator, the inner end, of theinsulator being of cup-shape adjacent to, and

openv toward, the inner end of the pplug and a length of the insulatorconstituting a substantial length of the plug protruding fromtheouterend of the shell having convex exterior surface curvaturerenderingthesame of gradual taper converging from aposition adjacent tothe shell to a position adjacent to the .outerend of the plug.

5. In a spark plug, an inner electrode, an outer metal shell, andinsulating meanssurrounding said electrode in the shell havingan'outer-end metal shell, and a one-piece insulator surroundelectrode inthelshell having an outer length protruding from the shell, saidinsulator means extending continuously from said length to aninsulatingcup portion thereof at the inner end of the plug,-andthe inner end of.the'shell surrounding said cup in spaced relationthereto andhaving itssmallest inside diameter no less than the largest inside diameter of thecup.

6. In a spark plug, an insulator having an inner-end cup portion opentoward the inner end of the plug, an outer metal shell including aportion enclosing a free space around said cup portion having aradially-inwardly extending peripheral flange spaced longitudinallyinwardly of the plug from the open end edge of the cup and leaving thecup substantially fully open, and an inner electrode extending throughthe insulator free from lateral protuberances adjacent to said flange.

7. In a spark plug, an insulator having an inner-end cup portion opentoward the inner end of the plug, an outer metal shell including aportion enclosing a free space around said cup portion having aradially-inwardly extending substantially fiat-plane peripheral flangespaced longitudinally inwardly of the plug from the open-end edge of thecup and leaving the cup substantiallyfully open, and an inner electrodeextending through and beyond the insulator and terminating close to theplane of said flange.

8. In a spark plug, an insulator having an in ner-end cup portion opentoward the inner end of the plug, an outer metal shell including aportion enclosing a free space around said cup portion having aradially-inwardly extending longitudinally-inextensive peripheral flangespaced longitudinally inwardly of the plug from the open-end edge of thecup and leaving the cup substantially fully open, and an inner electrodeextending through the insulator and through said flange and terminatingclose to the latter.

9. In a spark plug, an inner electrode, an insulator surrounding saidelectrode having an innerend cup portion open toward the inner end ofthe plug, and an outer metal shell including a portion surrounding saidcup portion in radiallyspaced relation thereto having aradially-inwardly extending longitudinally inextensive peripheral flangespaced longitudinally inwardly of the plug beyond the open-end edge ofthe cup, said flange so cooperating with said electrode and having itsinner diameter such as to leave the mouth of the cup substantially fullyopen to the arcing space except for the space occupied in the cup by theelectrode.

10. In a spark plug, an inner electrode, an insulator surrounding saidelectrode having a body and an inner end length of smaller diameter thansaid body portion including a cup portion freely open to the arcingspace and confined entirely within said length having an inner surfaceof frusto-conical form having its open-end major base at the innerend ofthe insulator and its minor base only slightly greater in diameter thanthe diameter of said electrode and spaced a substantial distance fromsaid body, the inner surface of said cup being entirely insulatingbetween said major and minor bases, an outer metal shell surrounding,and spaced from, said cup, and a ground electrode on the shellcooperating wit said inner electrode outside said cup.-

11. In a spark plug, an inner electrode, an outer metal shell, and aone-piece insulator surrounding said electrode in the shell having anouter end length protruding from the shell and an inner end cup entirelylocal to the inner end of the insulator enclosing a substantial freespace volume around and extending radially to said electrodesubstantially fully open around the electrode directly to the spaceopposite the inner end of the plug, said shell including a portionenclosing a free space around said cup and having its inner end disposedclose to the inner end of the cup.

12. In a spark plug, an inner electrode, an insulator surrounding saidelectrode having an inner-end cup portion open toward the inner end oithe plug, and an outer metal shell including a portion surrounding saidcup portion in radially-spaced relation thereto having ground electrodemeans and a radially inwardly extending iongitudinally inextensiveperipheral flange spaced longitudinally inwardly from the open end edgeof the cup, said flange and ground electrode means cooperating with saidinner electrode to maintain the interior cup space around the innerelectrode substantially fully open directly to the space opposite theinner end of the plug.

13. In a spark plug, inner electrode means, an outer metal shell havingground electrode means, and a one-piece insulator surrounding said innerelectrode. means in the shell including an exposed. outer end lengthprotruding from the shell and a short hollow portion local to the innerend of the insulator having substantial inside free space volume, saidshell having a length surrounding said hollow portion in laterallyspaced relation thereto with the inner end of the shell close to theinner end of said hollow portion and cooperating with said inner andground electrode means to maintain the inner end of said hollow portionsubstantially fully open around the inner electrode means directly tothe space opposite the inner end of the plug.

14. In a spark plug, inner electrode means, an outer metal shell havingground electrode means, and a one-piece insulator surrounding said innerelectrode means in the shell including an exposed outer end lengthprotruding from the shell and a short hollow portion local to the innerend of the insulator closed at its outer end enclosing substantial freespace volume about, and extending laterally to, said inner electrodemeans, an inner end length of the shell enclosing a free space aroundsaid hollow portion with its inner end close to the inner end thereofand cooperating with said inner and ground electrode means to maintainsaid hollow portion substantially fully open around the inner electrodemeans directly to the space opposite the inner end of the plug.

15. In a spark plug, inner electrode means, an outer metal shell havingground electrode means, and a one-piece insulator surrounding said innerelectrode means in the shell having an outer end length protruding fromthe shell and a short length of substantially cup-shape local to theinner end of the insulator defining free space of substantial volumeextending laterally from the cup side walls to said inner electrodemeans and substantially fully open around said inner electrode means tothe area about the inner end of the plug, said inner and groundelectrode means cooperating with each other to form a current paththrough the mouth of the cup.

16. In a spark plug, an outer metal shell having ground electrode means,inner electrode means entirely of conducting material from the outerplug end terminal means to the inner arcing space, and insulator meanssurrounding said inner electrode means in the shell including a plugclosed at its outer end enclosing substantial freesp'ace'volume about,and extending laterally to; said inner electrode means, the openend ofsaid short hollow portion beingclose to the innermost end of the plugandsaid inner and ground electrode means cooperating with each other toform current path means through said openend and to maintainsaid openend substanmany fully open around said inner electrode means directly tothe area opposite the inner end or: the plug.

17. Ina sparli'plug; an outer metal shell having "ground electrodemeans, inner electrode means; and'insulato'r means adapted for only onesnore hollow'portion-local to' tlie inner end of the pes iti'on in-thetime sur' ounfiin said 'i'nrier lec hollow portiombeing closeto-the-innermost end of the plug' and said inner and ground electrodemeans=cooperating with each other to form-cup rent. path means throughsaid open end and to maintain said open end substantially fully openaround saidinner electrode means directly to the area opposite the innerend of the plug.

CARL BERSTLER.

